Fig. 1. Schematic of structure model. (a) Three-dimensional stereogram; (b) two-dimensional plane picture

Fig. 2. (a) Reflection spectra with horizontal slits of 460 nm; (b) reflection spectra after stretching two horizontal slits

Fig. 3. Coupling between slits along x direction and y direction

Fig. 4. Electric field distribution in symmetrical state. (a) Mode 1; (b) mode 2; (c) mode 3

Fig. 5. Electric field distribution in asymmetric state. (a) Mode 1; (b) mode 2; (c) mode 3; (d) mode 4; (e) mode 5

Fig. 6. (a) Reflection spectra after increasing period; (b) reflection spectra after increasing middle groove